diff --git a/document_en/maxflow.md b/document_en/maxflow.md
index d2c39f8..950c0b8 100644
--- a/document_en/maxflow.md
+++ b/document_en/maxflow.md
@@ -50,6 +50,7 @@ It adds an edge oriented from the vertex `from` to the vertex `to` with the capa
 **@{keyword.constraints}**
 
 - $s \neq t$
+- $0 \leq s, t \lt n$
 - The answer should be in `Cap`.
 
 **@{keyword.complexity}**
diff --git a/document_ja/maxflow.md b/document_ja/maxflow.md
index 119b8ce..a5cdc95 100644
--- a/document_ja/maxflow.md
+++ b/document_ja/maxflow.md
@@ -50,6 +50,7 @@ int graph.add_edge(int from, int to, Cap cap);
 **@{keyword.constraints}**
 
 - $s \neq t$
+- $0 \leq s, t \lt n$
 - 返り値が `Cap` に収まる
 
 **@{keyword.complexity}**